Studies of microbial biosynthetic and catabolic pathways provide major insights about enzyme mechanism, specificity, structure, regulation, and evolution. Harnessing these insights and the individual enzymes has many practical uses ranging from the synthesis of new drug compounds to bioremediation. This application will address two questions: how the C ring of the antibiotic antitumor agent tomaymycin is made and how polycyclic aromatic hydrocarbons (PAHs) are degraded by one major bacterial pathway. Both questions arose in the course of our studies on 4-oxalocrotonate tautomerase (4-OT) and other tautomerase superfamily members. 4-OT catalyzes a proton transfer reaction in the meta-fission pathway for the degradation of aromatic hydrocarbons. TomN, a newly discovered 4-OT homologue, led us to the tomaymycin pathway because it reportedly catalyzes a proton transfer reaction using a very different substrate. The second question emerged from our extensive use of acetylene compounds as mechanistic probes and inhibitors of 4-OT and other superfamily members. The answer to the first question advances our understanding of the biosynthesis of a large and highly diverse group of natural products, the pyrrolo[1,4]benzodiazepines that are important sources of antimicrobial and anticancer drugs and drug leads. The answer to the second question advances our understanding of PAH degradative pathways, notably ones used for the degradation of higher molecular weight PAHs, which, along with the lower molecular weight PAHs, are highly toxic and persistent environmental contaminants. Using a combination of mechanistic, biochemical, structural, and kinetic techniques, these questions will be addressed in three specific aims. These aims are to: (1) identify the reactions that convert L-dopa to the C-ring of tomaymycin; (2) identify the key elements of the TomK mechanism; and (3) establish the mechanism and determinants of specificity for the hydratase/aldolases in the naphthalene, phenanthrene, and fluoranthene degradative pathways. The results will also address our long- term goals, which are to obtain a more comprehensive understanding of the relationship between structure and function in enzyme-catalyzed reactions and to delineate the factors that govern protein structure, substrate recognition, and reaction specificity.